The distributors resorted to rotational load shedding of about 200-300 MW in their respective areas.

Large parts of Mumbai reeled under severe power outage on Tuesday after a 500 MW electricity generation unit of Tata Power Company in Trombay tripped at 9.45 am.

Areas such as Dharavi, Sion, Dadar, Mahim, Prabhadevi, Parel, Sewri, Byculla, Girgaon, Mumbai Central, parts of Hutatma Chowk and Ballard Pier in the island city; Bandra Kurla Complex, Santacruz, Saki Naka and Juhu in the western suburbs; and Chembur, Tilak Nagar, Ghatkopar and Vikhroli in the eastern suburbs had to bear the brunt of the power outage for most part of the day. Power supply was restored at several places by evening.

Efforts to restore the affected unit number 5 and to start production from the cold standby unit number 6 were still underway. Power supply is expected to be normal by late Tuesday night.

Earlier in the day, attempts by the power distribution companies to buy power from outside the city failed due to strain on transmission corridor, which connects Mumbai to other power-generating units in the state. The distributors resorted to rotational load shedding of about 200-300 MW in their respective areas. By late evening, the quantum of load-shedding reduced to about 70-90 MW, officials said.

Maharashtra State Electricity Distribution Company Ltd (MSEDCL), which supplies electricity to the rest of Maharashtra, has a standby agreement with Tata Power Company to supply power to Mumbai in the event of any shortfall. “We are in a position to supply the required quantum, but we could supply only 250 MW due to a constrained transmission corridor,” said Ram Dotonde, spokesperson of the MSEDCL.

The Brihanmumbai Electric Supply & Transport (BEST), which distributes power in island city, too, cited the same reason.

BEST’s Deputy Public Relations Officer Manoj Varade said the failure of unit number 5 had resulted in the non-availability of 255 MW from Tata Power Company. BEST has a standby arrangement of 255 MW with MSEDCL for which standby charges of Rs 9.11 crore are paid every month, he said. “However, due to transmission line constraint, the 255 MW power could not be brought in to Mumbai through the standby arrangement,” he added.

An official of BEST said the transmission corridor has the capacity to handle about 1,400 to 1,500 MW and cannot take additional load.

The average demand for power in Mumbai is about 3,000 MW of which about 900 MW demand comes from the island city, where BEST distributes power.

Tata Power does not supply electricity to Reliance Energy, but still many areas in Mumbai’s suburbs that fall under its jurisdiction were affected by the outages. “Only those places under Reliance Energy’s jurisdiction were affected where the transmission system is interconnected with Tata Power’s systems. The supply would have been normal if it was possible to get about 90 MW from outside,” a source said.

Meanwhile, Tata Power was working on restoring power generation capacity at the affected plant in Trombay. By 5 pm, power was partially restored through hydro power stations. The company also started initiating its standby cold unit number 6, which runs on oil.

According to the company’s investor presentation for July 2014, the company has a generation capacity of 1,580 MW at the Trombay power station.